So... what does it do that makes it worth 500$ (aside from being rare?)

Well, rarity aside it has a pretty unique filter (like an auto wah) that can be manipulated by the footpad. More importantly it lets you use the foot pad to crossfade between effects plugged into loops. Imagine crossfading your signal from screaming wah/fuzz to clean phaser (or whatever) by rocking your foot on the pad.

At least that's what I gather - I've never played through one but am pretty familiar with how Emmett used it on Parallel Galaxy

Incredibly unique and innovative when it was invented and - shockingly - nobody has really made anything like it (in design and functionality) since.

Eric

I traded a guy a bass fuzz for one,worth about 75 bucks.
I lucked out,just saying

Man you sure did! This one has been ended and relisted with a BIN of $$995. CRAZY! Too bad, it's a sweet little gizmo and I'd love to mess around with one